The present invention relates to a semiconductor device which is equipped with a resistor of polycrystalline silicon.
In recent years, it has become necessary to form a resistor of high resistance value, with a high degree of integration. In the technical field of the analog-to-digital signal conversion, for example, a CR filter which has a resistor of high resistance value is sometimes used for increasing the time constant of the filter. In this case, when the resistor is formed in a semiconductor substrate with an impurity region, its coefficient of voltage vs. resistance becomes large. Moreover, the frequency characteristics has large fluctuations because there is a large capacitance due to the PN junction between the impurity region of the resistor and the substrate. On the other hand, when the high resistance resistor is made of a polycrystalline film on the semiconductor substrate, the impurity concentration in the polycrystalline film must be restricted within a very small quantity to realize the high resistance value. Therefore, in this case, the sheet resistance of such a resistor varies widely according to the fabrication conditions. The controllability of the CR time constant is restricted as a matter of course.
It is, therefore, sufficient to make the resistor of polycrystalline silicon. The quantity of impurity introduced into the silicon is increased to an extent that it does not vary much with the fabrication conditions, and does not increase the total length of the resistor so that a desired resistance may be obtained. In this case, however, a large resistor-forming area is required on the semiconductor substrate so that a highly or densely integrated semiconductor device cannot be realized.
In the semiconductor device of the prior art, more specifically, a plurality of unit resistor elements are made in a single step, from a layer of polycrystalline silicon. The resistors are arranged parallel to one another over an insulating film overlying a semiconductor substrate. Their end portions are connected together by means of a metal film to provide a single resistor. Or, a single polycrystalline silicon layer is formed with a plurality of bends over the insulating film such that longer portions thereof extend parallel to one another to provide a resistor.
In a single patterning step in the etching process, i.e., in the photo-resist (PR) step, however, the spacing required between adjacent unit resistor elements or between the parallel longer portions is at least, for example, 2 to 5 .mu.m. This spacing is determined not only by the material and fabrication accuracy of the mask, the thickness of the photo-resist, the exposure conditions, the thickness of the polycrystalline silicon layer and so on, but also by the whiskers which could form from one polycrystalline silicon pattern to an adjoining polycrystalline silicon pattern, namely, fine projections of polycrystalline silicon that may be undesirably formed by one PR step. If one of these fine projections comes into contact with the adjoining polycrystalline silicon pattern, a short-circuiting will naturally take place, to vary the resistance. This makes it necessary to enlarge the spacing.
Thus, the prior art has failed to provide a highly or densely integrated semiconductor device because of that limit in the spacing.